Cephalexin (Beta-Lactam Antibiotic): Appropriate Use and Dosage
Cephalexin is a first-generation cephalosporin antibiotic recommended at a standard adult dosage of 250-500 mg every 6 hours orally (1-4 g daily in divided doses) for treating bacterial skin and soft tissue infections, with dosing adjusted based on infection severity and patient characteristics. 1
General Information and Mechanism
- Cephalexin is a beta-lactam antibiotic in the first-generation cephalosporin class that is effective against many gram-positive bacteria, particularly Staphylococcus aureus (methicillin-susceptible) and Streptococcus species 2
- It is rapidly absorbed in the upper intestine (not the stomach) and achieves good tissue distribution except in cerebrospinal fluid and aqueous humor 2
- Approximately 70-100% of the dose is excreted unchanged in the urine within 6-8 hours, making it effective for urinary tract infections 2
FDA-Approved Dosing for Adults
Standard Dosing:
- General adult dosage range: 1-4 g daily in divided doses 1
- Usual adult dose: 250 mg every 6 hours orally 1
- For streptococcal pharyngitis, skin/skin structure infections, and uncomplicated cystitis: 500 mg every 12 hours orally 1
- For more severe infections: Larger doses may be needed, but if daily doses greater than 4 g are required, parenteral cephalosporins should be considered 1
FDA-Approved Dosing for Pediatric Patients
- Standard pediatric dosage: 25-50 mg/kg/day in divided doses 1
- For streptococcal pharyngitis (patients >1 year) and skin/skin structure infections: Total daily dose may be divided and administered every 12 hours 1
- For otitis media: 75-100 mg/kg/day in 4 divided doses (based on clinical studies) 1
- For β-hemolytic streptococcal infections: Treatment should continue for at least 10 days 1
Specific Clinical Applications
Skin and Soft Tissue Infections
- For impetigo: 250 mg four times daily (adults) or 25-50 mg/kg/day in 3-4 divided doses (children) 3
- For cellulitis without purulent drainage: Cephalexin is effective as monotherapy since β-hemolytic streptococci are the presumed predominant pathogens 4
- For surgical site infections (trunk or extremity away from axilla or perineum): 500 mg every 6 hours orally 3
- For methicillin-susceptible S. aureus (MSSA) skin infections: 500 mg four times daily orally 3
Upper Respiratory Tract Infections
- For streptococcal pharyngitis/tonsillitis: 500 mg every 12 hours for adults 1, 5
- For otitis media in children: Higher dosages of 75-100 mg/kg/day in 4 divided doses are required 1, 5
Special Considerations
Renal Impairment
- Patients with creatinine clearance less than 30 mL/min require dose reduction proportional to the reduced renal function 2
Duration of Therapy
- For skin and soft tissue infections: Typically 7 days, depending on clinical response 3
- For streptococcal infections: Minimum 10-day course to prevent rheumatic fever 1
- For uncomplicated cystitis: 7-14 days 1
Comparative Efficacy
- Cephalexin has similar in vitro activity against MSSA compared to cefadroxil (MIC50 values of 2 μg/mL and MIC90 values of 4 μg/mL for both) 6
- In the treatment of uncomplicated cellulitis, cephalexin alone showed similar clinical cure rates to combination therapy with cephalexin plus trimethoprim-sulfamethoxazole 4
- For staphylococcal skin infections, twice-daily dosing with cephalexin has been shown to be as effective as four-times-daily dosing with dicloxacillin 7
Common Adverse Effects
- Primarily gastrointestinal upset, rash, and urticaria 5
- Adverse effects are relatively infrequent and rarely require discontinuation of the drug 5
- Low incidence of allergic reactions due to its stability and chemical configuration 2
Important Limitations
- Not effective against methicillin-resistant Staphylococcus aureus (MRSA) 3
- Limited efficacy against Haemophilus influenzae infections (approximately 50% failure rate) 5
- Not recommended for infections requiring CSF penetration due to poor blood-brain barrier crossing 2